The present invention relates to imaging properties of tissue based upon differential absorption of electromagnetic waves in differing tissue types by photo-acoustic techniques.
It is well established that different biologic tissues display significantly different interactions with electromagnetic radiation from the visible and infrared into the microwave region of the electromagnetic spectrum. While researchers have successfully quantified these interactions in vitro, they have met with only limited success when attempting to localize sites of optical interactions in vivo. Consequently, in vivo imaging of disease at these energies has not developed into a clinically significant diagnostic tool.
In the visible and near-infrared regions of the electromagnetic spectrum, ubiquitous scattering of light presents the greatest obstacle to imaging. In these regions, scattering coefficients of 10-100 mmxe2x88x921 are encountered. Consequently, useful numbers of unscattered photons do not pass through more than a few millimeters of tissue, and image reconstruction must rely on multiply-scattered photons. While efforts persist to use visible and infrared radiation for imaging through thick tissue (thicker than a few centimeters), clinically viable imaging instrumentation has not been forthcoming.
In the microwave region (100-3000 MHZ), the situation is different. Scattering is not as important, since the wavelength (in biologic tissue) at these frequencies is much greater than the xe2x80x9ctypicalxe2x80x9d dimension of tissue inhomogeneities (≈1 xcexcm). However, the offsetting effects of diffraction and absorption have forced the use of long wavelengths, limiting the spatial resolution that can be achieved in biologic systems. At the low end of the microwave frequency range, tissue penetration is good, but the wavelengths are large. At the high end of this range, where wavelengths are shorter, tissue penetration is poor. To achieve sufficient energy transmission, microwave wavelengths of roughly 2-12 cm (in tissue) have been used. However, at such a long wavelength, the spatial resolution that can be achieved is no better than roughly xc2xd the microwave length, or about 1-6 cm.
In vivo imaging has also been performed using ultrasound techniques. In this technique, an acoustic rather than electromagnetic wave propagates through the tissue, reflecting from tissue boundary regions where there are changes in acoustic impedance. Typically, a piezoelectric ceramic chip is electrically pulsed, causing the chip to mechanically oscillate at a frequency of a few megahertz. The vibrating chip is placed in contact with tissue, generating a narrow beam of acoustic waves in the tissue. Reflections of this wave cause the chip to vibrate, which vibrations are converted to detectable electrical energy, which is recorded.
The duration in time between the original pulse and its reflection is roughly proportional to the distance from the piezoelectric chip to the tissue discontinuity. Furthermore, since the ultrasonic energy is emitted in a narrow beam, the recorded echoes identify features only along a narrow strip in the tissue. Thus, by varying the direction of the ultrasonic pulse propagation, multi-dimensional images can be assembled a line at a time, each line representing the variation of acoustic properties of tissue along the direction of propagation of one ultrasonic pulse.
For most diagnostic applications, ultrasonic techniques can localize tissue discontinuities to within about a millimeter. Thus, ultrasound techniques are capable of higher spatial resolution than microwave imaging.
The photoacoustic effect was first described in 1881 by Alexander Graham Bell and others, who studied the acoustic signals that were produced whenever a gas in an enclosed cell is illuminated with a periodically modulated light source. When the light source is modulated at an audio frequency, the periodic heating and cooling of the gas sample produced an acoustic signal in the audible range that could be detected with a microphone. Since that time, the photoacoustic effect has been studied extensively and used mainly for spectroscopic analysis of gases, liquid and solid samples.
It was first suggested that photoacoustics, also known as thermoacoustics, could be used to interrogate living tissue in 1981, but no subsequent imaging techniques were developed. The state of prior art of imaging of soft tissues using photoacoustic, or thermoacoustic, interactions is best summarized in Bowen U.S. Pat. No. 4,385,634. In this document, Bowen teaches that ultrasonic signals can be induced in soft tissue whenever pulsed radiation is absorbed within the tissue, and that these ultrasonic signals can be detected by a transducer placed outside the body. Bowen derives a relationship (Bowen""s equation 21) between the pressure signals p(z,t) induced by the photoacoustic interaction and the first time derivative of a heating functions, S(z,t), that represents the local heating produced by radiation absorption. Bowen teaches that the distance between a site of radiation absorption within soft tissue is related to the time delay between the time when the radiation was absorbed and when the acoustic wave was detected.
Bowen discusses producing xe2x80x9cimagesxe2x80x9d indicating the composition of a structure, and detecting pressure signals at multiple locations, but the geometry and distribution of multiple transducers, the means for coupling these transducers to the soft tissue, and their geometrical relationship to the source of radiation, are not described. Additionally, nowhere does Bowen teach how the measured pressure signals from these multiple locations are to be processed in order to form a 2- or 3-dimensional image of the internal structures of the soft tissue. The only examples presented are 1-dimensional in nature, and merely illustrate the simple relationship between delay time and distance from transducer to absorption site.
The above-referenced U.S. Pat. No. 5,713,356, filed by the present inventor, details a diagnostic imaging technique in which pulses of electromagnetic radiation are used to excite a relatively large volume of tissue and stimulate acoustic energy. Typically, a large number of such pulses (e.g., 100 to 100,000), spaced at a repetition interval, are generated to stimulate the tissue. The above-referenced U.S. Pat. No. 5,713,356 discloses methods for measuring the relative time delays of the acoustic waves generated by a sequence of such pulses, and for converting these time delays into a diagnostic image.
In one aspect, the invention features a thermoacoustic imaging system including an electromagnetic radiation source for irradiating said tissue to stimulate a thermoacoustic response, a coupling media for acoustically coupling the response to an acoustic sensor array, and an acoustic sensor array. The array comprises sensors arranged on a surface, which is rotatable about an axis to position said sensors in a plurality of positions for detecting the thermoacoustic response. The angular extent of the surface about the axis, subtends an angle that is less than a full revolution, so that the surface is substantially smaller than the sensor bowl described in the above-referenced U.S. Pat. No. 5,713,356, streamlining the device and permitting greater flexibility in positioning the radiation source and other elements of the device.
In the described specific embodiment, the array surface is made of a plurality of flat sections, in an arc, arranged such that a geometric center of each section is equidistant from a common point on the axis of rotation. Each section each carries a plurality of acoustic sensors, the sections nearer to the axis of rotation carrying fewer acoustic sensors than the sections further from that axis.
The acoustic sensors may comprise singe piezoelectric sensors, or dual sensors arranged side-by-side or coaxially with a combiner for combining their signals to form a combined signal.
A sensor array such as described may also be used, in conjunction with an ultrasound beam steering circuit, as an ultrasonic imaging device. Specifically, the beam steering circuit is coupled to the acoustic sensors and stimulates the sensors to produce an ultrasound beam directed into said tissue. Echoes of this beam are received by the sensors and combined to form an image of the tissue.
In a second aspect, the invention features a thermoacoustic imaging system, in which the source of electromagnetic radiation for irradiating the tissue is a plurality of sources arranged about the tissue and producing synchronized electromagnetic radiation in varying polarizations or phases to irradiate said tissue with electromagnetic radiation of a desired polarization.
In the described specific embodiment, the sources comprise waveguides positioned to launch electromagnetic radiation toward said tissue, in varying polarizations or phases, such as vertical and horizontal polarization and/or relative phase shifts of zero and ninety degrees.
In a further aspect, the invention features a handheld thermoacoustic imaging system for imaging structures of tissue. Specifically, thermoacoustic imaging is performed with a device sized to be held within the human hand, which has a source of electromagnetic radiation for irradiating the tissue to stimulate a thermoacoustic response, and an acoustic sensor array for detecting the thermoacoustic response.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.